1. Technical Field
The present invention relates to the field of metrology, and more particularly, to focus measurements using scatterometry metrology.
2. Discussion of Related Art
The lithographic tool is a machine that is used to print a desired pattern onto substrate. The tool is used to transfer a pattern from a mask to an individual layer of the integrated circuit, printed on a wafer. The transfer is typically carried out via imaging onto a sensitive layer, termed the resist. As the targeted critical dimension (CD) of the pattern elements shrinks, the imaging process window shrinks, which results in a smaller process window in terms of depth of focus (DOF). In order to control the printed pattern uniformity, it is necessary to measure the parameters of the lithographic tool and especially the parameters of the focus. For example, the advanced node requires very tight focus control, e.g., 3σ<10 nm across the wafer.
Wong et al. 2013 (“CD optimization methodology for extending optical lithography” in Proc. of SPIE Vol. 8681, 868137), the article being incorporated herein by reference in its entirety, teach using, for focus measurements, asymmetric targets for which the asymmetry, i.e., the difference between the effective side wall angles (SWA) at the left and the right edges, changes monotonically through focus (see reference 1). The SWA difference shows up as an intensity difference between +1 and −1 diffractions orders in the measured scatterometry signal. However, the target pitch taught by Wong et al. is at least four times the product pitch, which makes the target sensitive to lithographic tool aberration. Another issue is that when the SWA angle asymmetry is small the signal difference becomes small, leading to inaccurate results.
Spaziani et al. 2012 (“Lithography process control using in-line metrology” in Proc. of SPIE Vol. 8324, 83241L) and Brunner and Ausschnitt 2007 (“Process Monitor Gratings” in Proc. of SPIE Vol. 6518-2), the articles being incorporated herein by reference in their entirety, teach using, for focus measurements, Focus Dose Pattern and Process Monitor Grating, in which targets are designed for higher sensitivity to dose and focus variation. The targets are made more sensitive by using scattering bar techniques, end of line techniques, and forbidden pitch. However, the targets are disadvantageous with respect to their small depth of field (DOF) and the printability of the patterns in a whole range of process window. The measurement method uses the scatterometry model base approach which makes it sensitive to model errors.
U.S. Patent Publication No. 20140141536, which is incorporated herein by reference in its entirety, discloses a segmented mask including a set of cell structures. Each cell structure includes a set of features having an unresolvable segmentation pitch along a first direction. The unresolvable segmentation pitch along the first direction is smaller than the illumination of the lithography printing tool. The cell structures have a pitch along a second direction perpendicular to the first direction. The unresolvable segmentation pitch is suitable for generating a printed pattern for shifting the best focus position of the lithography tool by a selected amount to achieve a selected level of focus sensitivity.